Photolithography is a technique for fabricating devices such as a semiconductor device, a liquid crystal display device, an image capturing device (e.g., a charge coupled device, i.e., a CCD), or a thin film magnetic head and the like. Generally, photolithography involves transferring a pattern from a pattern area on a photomask to the surface of a thin film or the bulk of a substrate coated with a photoresist material. This is accomplished by aligning the coated substrate with the photomask and then exposing the aligned components repeatedly to light. The wavelength of light used depends on the composition of the photoresist material. If foreign matter such as dust or other particulates comes to rest on the pattern area of the photomask, the resulting pattern on the substrate will very likely be damaged and the substrate defective and unusable.
A protective apparatus, known as a pellicle, is often affixed to the photomask to cover the pattern area and prevent foreign matter from settling on, or remaining floating above, the pattern area during the manufacture process. The pellicle has a pellicle frame comprising four walls which define a trapezoidal area that is substantially commensurate with the pattern area on the photomask to be protected. The frame is made of a material that is semi-rigid, as well as opaque and inert to light, including aluminum, aluminum alloys, stainless steel, polyethylene, and similar materials. The pellicle also includes a pellicle film affixed to an upper edge of the frame and extending across the area defined by the frame.
At least one vent hole is typically provided in at least one of the frame walls to allow for equalizing the pressure between air contained in a space created between the photomask surface and a pellicle affixed thereto and ambient air surrounding the photomask. A filter is often used to cover the vent hole for preventing foreign matter from entering the vent hole and settling on, or floating above, the pattern area.
In the device fabrication process that produces semiconductor wafers, a simple surface air spray, such as nitrogen air gun spray, may be performed to enhance the entire photomask substrate surface cleanness. This procedure, however, sometimes introduces foreign matter into the space between the photomask and the pellicle, through the vent hole, where it may undesirably settle on or float above the pattern area of the photomask. To prevent such introduction of foreign matter through the vent hole, pellicle manufacturers typically caution that the nitrogen or air source should be at least 10 millimeters away from the filter covering the vent hole, and that the pressure of the nitrogen or air must be 0.20 megapascals (MPa) or less, and that the duration of blowing the nitrogen or air must be less than 10 seconds. Nonetheless, even adherence to these precautions does not guarantee no foreign matter will enter through the vent hole and filter. It has been found that the problem of foreign matter entering through the vent hole and filter occurs more than most other manufacturing issues that arise when using pellicles with photomasks. Moreover, testing has shown that when floating particles were inadvertently introduced through the vent hole and under the pellicle film, most of the floating particles were located close to the vent hole. Thus, resolution of this issue clearly merits particular attention.
Moreover, the pellicle also has a mask-side edge located opposite the upper edge to which the pellicle film is affixed. The pellicle is typically affixed to the photomask with an adhesive between the mask-side edge and the photomask. Unfortunately, when the thickness of the frame walls is uniform from the mask-side edge to the pellicle film-side edge, it is often not possible to inspect the entire trapezoidal area of the photomask defined by the frame because a peripheral portion of the area proximate the mask-side edge of the frame is obstructed by shadow cast by the frame walls. This is undesirable since the devices being manufactured are typically very small and, therefore, the useful and accessible area on these devices must be maximized to maximize their value and efficiency. Furthermore, pattern areas on a photomask that are obstructed and cannot be inspected also cannot be subjected to quality control and guaranteed to function as intended, which means such obstructed areas are not useful, which diminishes the efficiency and value of the photomask.
Accordingly, it is desirable to provide a pellicle having improved ability to prevent foreign matter from settling on, or floating above, the pattern area of a photomask. It is also desirable to provide a pellicle having a larger unobstructed, inspectable area defined by the pellicle frame. Furthermore, other desirable features and characteristics will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the following background.